To elaborate: RCS, CVS, Subversion and Git (moreover Hg, etc.) represent a generation of Version control, each revolutionizing the way we work with source control.

RCS introduced version control

CVS introduced version control with code exchange on a server

SVN introduced version control with atomic committing and repository wide file versions (as opposed to file-specific revisions).

To me it seems that HG is in that sense very close to Git. However I do wonder what might be the next revolution

The Git/Hg generation popularized decentralized version control and made code-sharing easier than ever.

Not quite. It appeared about the same time as git and Hg already lost its battle against git for becoming The VCS Of The Choice[tm].

Even then, Mercurial works in about the same operation model as git.